hgweb: partially revert 4ec58b157265
The change from self.repo.ui.diffopts() to ui.diffopts() causes a
traceback when the diff of a changeset should be displayed in the
web interface. Revert to self.repo.ui.diffopts() for now.
#header#
<title>Mercurial Error</title>
</head>
<body>
<h2>Mercurial Error</h2>
<p>
An error occured while processing your request:
</p>
<p>
#error|escape#
</p>
#footer#